Heretofore, Fresnel lenses have been of two types, a flat type and a curved convex type. A three-dimensional or spherical Fresnel lens is designed to focus on a point, and a two-dimensional or cylindrical lens is designed to focus on a line.
Previously, Fresnel lenses have been primarily designed for use with a point source of light to create a wide collimated beam of light such as those used in a lighthouse or studio stagelight.
An example of the beam focusing lens is disclosed in the patent to Pascucci, U.S. Pat. No. 1,504,970, which discloses a Fresnel lens having on one face concentric zones united by miters, the miters being cut whereby they are parallel to the path of the rays passing through the lens. The juncture of the miters between the bottom face and the rear face of the prism is assumed to be a perfectly pointed configuration. However, in the manufacture of these types of lenses out of material such as glass or acrylic plastics the surface tension of the material placed in the mold at the juncture of these faces will cause the point to be rounded. If the mitered face is constructed, as taught by Pascucci, and aligned parallel to the path of the rays, and if the lens is used to focus sunlight, the rounded point will cause divergence of the light rays along that portion away from the focal point as the light passes through the prism. In addition, errors occur in manufacturing the prisms and in aiming of the lens toward the sun. These errors will cause a portion of the light to intercept the bottom face and thus refract away from the focal point. The loss due to light striking the rounded corner and bottom face could be anywhere from 20% when the lens is glass to 5 to 10% when the lens is an extruded plastic type lens.
Fresnel lenses have been used in solar collectors as disclosed by the patent to Sleeper, U.S. Pat. No. 3,125,091, which discloses an inflatable cylindrical type lens having prisms formed therein.
Prisms in the Sleeper patent are superimposed on a circular arc which imposes restrictions on the optical efficiency of this type of lens. The light passing through the edges of this type of circular lens is so severely deflected that it is completely lost. In addition, the Sleeper collector teaches a flexible type solar collector which is incapable of reaching high temperatures and withstanding exterior weather conditions for 20 years or more.
Heretofore, most prior art Fresnel lenses had longer focal lengths with F-numbers of 1.0 or greater. This required more material, more insulation and a greater volumne of space to construct the lens and solar collector to collect the proper amount of heat. Furthermore, much of the material content (and thus the cost) of the collector is due to the housing and structural support system, which can be greatly reduced by reducing the lens focal length to make a more compact collector unit. Unfortunately, conventional Fresnel-type lenses suffer great losses in transmittance as the focal length is reduced. This is the reason that prior-art Fresnel lens collectors have utilized long focal length lenses with F-Numbers (focal length divided by lens width) of 1.0 or greater.
It should be readily apparent that solar collectors must be highly efficient in order to utilize the full potential of converting the sun's energy into a useful form of energy. The loss of 10 or 20% of the energy transmitted through the lens is often times critical as to whether the system may be used for merely heating purposes or for conversion of light into energy useful for air conditioning, generation of electricity, or other processes. In the past, the flat plate collectors have been used due to the simplicity of construction. However, flat plate collectors need a large area and a large heat absorber, and have very low collection efficiencies. Heat absorbers are often constructed from stainless steel or copper to minimize corrosion and are therefore extremely expensive to manufacture.
Fresnel lenses in use now have a high loss of light transmittance through the lens due primarily to reflections at the prism surfaces. This high reflection loss causes a large decrease in collection efficiency.
Therefore, it is highly desirable to produce a solar concentrator which has the highest transmittance and the shortest possible focal length. Such a concentrator will achieve the highest collection efficiency at minimum cost. The new concentrator described below has these beneficial characteristics.